MCVD (Modified Chemical Vapor Deposition) is one of methods for manufacturing optical fibers. In MCVD, a clad is firstly formed and then a core is formed in the clad.
Referring to FIG. 1 for describing the conventional MCVD more specifically, a quartz tube 1 is put on headstocks of a lathe, and then a soot forming gas such as SiCl4, GeCl4 and POCl3 is introduced into the tube 1 together with oxygen gas while the tube 1 is rotated. At the same time, a torch 2 for providing a temperature above 1600° C. is reciprocated along an axial direction of the tube 1 so that the soot forming gas introduced into the tube 1 may be sufficiently reacted.
Whenever the torch 14 reciprocates, oxidation of halide gas is induced as expressed in the following Reaction Formula 1 at a region in the tube 1 that is heated up to a reaction temperature, so fine glass particles (hereinafter, referred to as ‘soot’) 3 are generated. While the torch 2 is moving, the soot 3 is deposited on the inner surface of the tube 1 by means of thermophoresis at a region that has a relatively lower temperature than the region heated by the torch 2.SiCl4+O2→SiO2+2Cl2GeCl4+O2→GeO2+2Cl2  Reaction Formula 1
The layer of soot 3 deposited on the inner surface of the tube 1 is sintered by the heat of the torch 2 adjacently followed and becomes a transparent glass layer. This process is continuously repeated so that a plurality of clad layers are formed on the inner side of the tube 1 and a plurality of core layers are subsequently formed on the clad layer. FIG. 2 shows a section of the optical fiber preform produced by the aforementioned process. In FIG. 2, reference numeral 5 denotes a core, 6 denotes a clad, 7 denotes a tube, 8 denotes a diameter of the core, and 9 denotes a diameter of the clad.
In the conventional MCVD however, while a plurality of clad layers and core layers are formed, there occurs a problem that hydroxyl groups (OH) are included therein as impurities. In fact, the soot forming gas flowed into the tube 1 generally contains a small amount of moisture as impurities, and this moisture is adsorbed on the surface of the deposition layer formed inside the tube 1 and then dispersed into the deposition layer under the high temperature, thereby causing bond of Si and OH. FIG. 3 shows an interatomic bond structure of the sintered soot deposition layer in case of the conventional optical fiber preform producing process using MCVD. Referring to FIG. 3, it would be found that a large amount of hydroxyl groups (OH) and Si are bonded therein.
However, since the depositing and sintering of the soot 3 are achieved substantially at the same time using the torch 2 in the MCVD according to the prior art, the removal of hydroxyl groups (OH) included in the clad layer or the core layer as impurities is substantially not possible unless a dehydration process is separately conducted. It is because the hydroxyl groups (OH) included as impurities in the soot 3 through chemical reaction are stably bonded to Si and thus stay in the soot 3 though the MCVD process is conducted at a high temperature.
Meanwhile, an optical loss, which is the most essential feature for optical fibers, is composed of a Rayleigh scattering loss caused by the difference of density and constitution of an optical fiber preform, an ultraviolet absorption loss according to electronic transition energy absorption in an atom level, an infrared absorption loss according to energy absorption during lattice vibration, a hydroxyl group absorption loss due to vibration of hydroxyl groups (OH), and a macroscopic bending loss.
The optical loss should be lowered in order to ensure reliable signal transmission through optical fibers. An optical fiber generally has an optical loss lower than a predetermined level in the wavelength range between 1100 nm and 1700 nm, so two wavelengths of 1310 nm and 1550 nm are currently used as main wavelength ranges for optical communication. In addition, the optical loss due to the hydroxyl group (OH) absorption is particularly considered as a significant factor at the wavelength 1385 nm rather than in other wavelengths, and this wavelength is at present not used due to the high optical loss caused by the hydroxyl group (OH) absorption. Thus, in order to use all of the wavelength range from 1310 nm to 1550 nm, an average optical loss caused by the hydroxyl group (OH) in the optical fiber at the wavelength 1385 nm should be lower than that of 1310 nm (0.34 dB/Km on average). Since the core composed of germanium dioxide and silicon dioxide has a Rayleigh loss of about 0.28 dB/Km caused by the density and constitution difference of the core material itself, the optical fiber can be used in the wavelength range from 1310 nm to 1550 nm only when the optical loss caused by hydroxyl groups (OH) is controlled to 0.06 dB/Km or below. For this purpose, the production of the optical fiber preform should be also controlled so that the concentration of hydroxyl groups (OH) in the optical fiber is not more than 0.8 ppb. However, when just two hydroxyl groups exist on the surface of a particle with a diameter of 0.1 μm, the concentration of hydroxyl group comes up to 30 ppb and this concentration may be converted into an optical loss of even 0.75 dB/Km. This fact shows that it is very difficult to control the concentration of hydroxyl groups (OH) contained in the optical fiber preform as impurities in the level of not more than 0.8 ppb in the conventional MCVD.
It is known that making an OH-free single mode optical fiber is possible in OVD (Outside Vapor Deposition) as disclosed in U.S. Pat. No. 3,737,292, U.S. Pat. No. 3,823,995 and U.S. Pat. No. 3,884,550, or in VAD (Vapor Axial Deposition) as disclosed in U.S. Pat. No. 4,737,179 and U.S. Pat. No. 6,131,415.
However, different to OVD and VAD, the conventional MCVD executes the deposition process and the sintering process at the same time, so soot is simultaneously melted and condensed while the soot is formed. Thus, in the optical fiber produced by the conventional MCVD, Si—OH included in the glass layer condensed due to the sintering causes a critical hydroxyl group (OH) absorption loss at the wavelength 1385 nm. Accordingly, the optical fiber drawn from the preform produced by the conventional MCVD has a limitation in the usable optical communication wavelength range.
Japanese Laid-open Publication S63-315530 discloses a method for producing an optical fiber preform, which includes the steps of accumulating metal oxide fine particles including SiO2 on an inner wall of a quartz tube to form a porous accumulative layer, flowing a dehydrating agent into the quartz tube with the porous accumulative layer to dehydrate the porous accumulative layer, making the porous accumulative layer into a transparent glass with flowing the dehydrating agent into the quartz tube, and condensing the quartz tube filled with the dehydrating agent.
S63-315530 conducts the dehydration process after the clad layer and the core layer are all accumulated in the quartz tube, so it is difficult to completely remove all hydroxyl groups (OH) existing in the deposition layer if the deposition layer (particularly, the core layer) is thick.
That is to say, S63-315530 is not suitable for producing an optical fiber having a larger preform and appropriate to common optical communication systems (particularly, CWDM) requiring the lowest absorption loss at 1385 nm.
In addition, the conventional dehydration process was directed to removing hydroxyl groups existing in the core layer. However, light passes through a part of the clad layer as well as the core layer. Thus, in order to reduce the absorption loss caused by hydroxyl groups to the minimum, it is needed to dehydrate all regions in MFD (Mode Field Diameter).